swap one material with another
Go to the Outliner. At the top click the button and switch to Blender File. Still in the Outliner choose Materials, twirl it open with the little triangle to the left. You will now see all the materials in the file in a list. Now you just need to select the offending materials, right click it, choose Remap Users. And from the list choose the correct material. Now they will all link to the same correct material.